1. Field of the Invention
The present invention relates to a storage device system used in information processing systems and computer systems, and especially to a virtual tape device and a virtual tape library device.
2. Related Background Art
Generally in computer systems, two types of storage media, magnetic disks and magnetic tapes, are used for data storage. Magnetic disks are devices in which data recorded at specified positions on magnetic disk devices can be directly accessed (i.e., random access), and are therefore used to record frequently used data. On the other hand, magnetic tapes are storage media in which data is written and read only in sequence from the beginning of the tapes (i.e., sequential access), and have therefore been used primarily for backup of data recorded on magnetic disks or for archiving old data to be stored long-term. The main reason for using magnetic tapes for backup and archiving purposes is that the bit cost of magnetic tapes is cheaper than that of magnetic disks, so that it costs less to store a large amount of data.
However, in recent years a rapid rise in the amount of data handled in computer systems has caused backup and restore processing to and from magnetic disks and magnetic tapes to take longer, such that making backups on magnetic tapes is beginning to be impractical. Backups are normally made when online operations do not take place, such as at night (off-line period); however, due to the fact that backups are taking too much time, there are instances where backup processing is not completed during off-line period. Furthermore, since the bit cost of magnetic disks are falling and approaching the bit cost of magnetic tapes, methods for using magnetic disks to store backup data are beginning to be considered.
For example, a device that uses a magnetic disk device and emulates a tape device has been proposed. With this device, when a command for the tape device is received from a host computer, the command is converted to a command for the magnetic disk, and data on the magnetic disk is accessed, such that the magnetic disk device can be used as a tape device. Such a device is called a virtual tape device.
The purpose of a virtual tape device is to be able to have such uses as making backups on virtual tape devices in existing computer environments that formerly used conventional tape devices, by replacing the conventional tape devices with virtual tape devices and without making any changes to host computers or backup software.
However, in such actual uses as making backups, a tape library device that maintains and manages a plurality of magnetic tapes also manages backups in many computer environments. In a backup operation to backup data that are distributed across a plurality of magnetic tapes, backup software operates the tape library device and thereby loads magnetic tapes into tape devices and replaces magnetic tapes without any human intervention. Consequently, in order to perform a backup processing that actually utilizes virtual tape devices in computer systems, a virtual library mechanism that realizes processing for loading virtual tape media into virtual tape devices and replacing virtual tapes is required.
The conventional technology teaches how to emulate a tape device but does not teach any technology regarding library mechanism, such as the operation of a library changer robot.
Furthermore, the virtual tape device only has functions of a tape device and cannot be used as a disk device. As a result, when making backups, data must be backed up on a virtual tape device from another disk device via a host computer. Consequently, even if the access speed of the magnetic disk is slightly faster than the access speed of the magnetic tape device, the backup speed does not improve significantly. Tape devices have reached higher read/write speeds especially in recent years, and this has made the performance of the tape devices comparable to the sequential read/write performance of disk devices. Consequently, even if a virtual tape device were used in place of a tape device, there would not be significant advantages in terms of performance.